Conventionally, to avoid pop generation by a loudspeaker of an audio system at power on or power off, an audio mute integrated circuit (IC) is adopted to eliminate pops. At power off, however, the audio mute IC lacks a sufficient supply voltage to support proper operation of its internal circuit, and is thus hard to maintain its mute function. To solve this problem, the supply voltage of the audio mute IC must be given with a longer sustained interval of time at power off so that after the power off, the internal circuit of the audio mute IC can still work normally for a period of time for the audio mute function to come into play, and the output signal of the audio system can still be properly maintained for a period of time.
U.S. Pat. No. 5,778,238 inserts a P-N junction diode between an external power source and a capacitor to charge the capacitor, so that the capacitor can provide energy for a low-voltage detector when the power source is removed, and thereby turns on a MOSFET to discharge a delay capacitor at the input terminal of a power on reset (POR) circuit. This will prevent the delay capacitor from providing a shorter delay time at the next power on. However, the diode connected between the external power source and the internal circuit leads to an additional voltage drop, and thus decreases the margin of the working voltage of the internal circuit. Moreover, the voltage outputted from the diode will fluctuate with the external supply voltage.